Wearing earbuds lets people witness the world around them while absolving them of some of the responsibility of responding to it. With earlier devices such as the Walkman and Discman, users had a limited number of songs, whereas digital devices give access to thousands. (iPod’s 40-gigabyte model holds up to 10,000 tunes.) There are songs for every mood, leading some to use music to create a buffer in which to move through the world, their own private digital ecosystem. “Urban spaces are rhythmically diverse, filled with different sounds and movements, many of them unpredictable,” says Bull. “Often spaces are rather anonymous or inhospitable or even hostile. iPod users make their world become in tune with them while, at the same time, becoming increasingly attentive to the relationship between music and their moods.”

Rather than technology driving this trend, Bull thinks the urge to cocoon is a social predisposition that the invention of portable digital music players lets people act on. (Not that technology is blameless. Andrew Jakubowicz, a sociologist at the University of Technology Sydney in Australia, argues that with so many people using cellphones, iPods are an antidote to the incessant chatter.)

As with any trend, there is no shortage of critics. Earlier this year, the conservative pundit Andrew Sullivan complained in the London Sunday Times about a “society without the social.” Admitting he uses an iPod himself, Sullivan nonetheless referred to “the sect of little white box worshippers” and went on to trash the phenomenon, making the argument that “technology has given us a universe for ourselves, where the serendipity of meeting a new stranger [or] hearing a piece of music we would never choose for ourselves or an opinion that might force us to change our mind about something are all effectively banished.”

A more thoughtful analysis was published in the winter 2005 issue of The New Atlantis, a technology and social journal. In “The Age of Egocasting,” writer Christine Rosen connected the TV remote to the current fetishistic attachment to a range of technology – including VCRs, web browsers, PDAs, iPods and TiVo – that lets individuals create customized content. Rosen defined “egocasting” as the “extremely narrow pursuit of one’s personal taste” and warned that it could lead to an ever more fragmented society.

The topic of customization also interests Clive Thompson, a New York-based technology writer for Wired, The New York Times Magazine and The Boston Globe. The trend is troubling, he says, when it means people consume only information sources that support their point of view. But when it comes to music, he thinks the gloomy hand wringing is excessive. Thompson subscribes to Rhapsody – a digital music service similar to Apple’s popular iTunes – which has a built-in collaborative-filtering system that recommends new music based on analysis of an individual’s past choices. “Music has always been tribal,” he says.